It is known that the best and easiest oilseed to process is soy bean. Rapeseed has also been processed on the kind of equipment used to process soy bean; however, it must be ground, cooked, and rolled into flakes to provide an extractable bed. Generally flakes can be run only at a much slower rate than soy beans. The resultant rapeseed residual material after oil removal, called "marc", cannot readily be used as meal for animal feeding due to the presence of trypsin inhibitors, high euric acid and high glucosinolates.
The process and apparatus of the present invention can be used to process oil seeds such as soy bean and safflower; however, the apparatus is particularly beneficial in obtaining oil from granular sources such as rice bran, wheat mill feed, rapeseed in general, and amaranth (as well as similar grains), which have lower oil content than soy bean and safflower and, except for rapeseed, have not been economically competitive as an oil source prior to the present process. Typically, oil seeds contain from about 15 percent to about 40 percent by weight oil. Most of the grains of the kind described above contain only from about 4 percent to about 40 percent by weight oil. Prior to the method of the present invention, processing of grain materials having this low oil content was not competitive with processing of oil seeds. The present oil extraction method makes possible not only the extraction of oil from grain materials, but further extraction of additional oils from crushed or flaked vegetable materials previously oil extracted using a less efficient method of extraction, such as crushed peanuts, cacao beans or olives. The present oil extraction method also enables additional oil extraction from grain-based products such as chips, noodles and crackers, by way of example and not limitation.
Edible oils are usually extracted from most oil seeds by soaking crushed or pulverized seeds in liquid hexane. Usually, the seeds are washed with hexane five to seven times to ensure the maximum recovery of oil. The amount of hexane required for such washing is quite high, up to 40 times the weight of oil recovered. The oil is then separated from the oil-hexane solution, or "miscella" by distillation or evaporation, and the hexane solvent is recovered for recycle use in the separation process. The marc that remains after removal of the oil from the seeds is generally a powder and contains substantial amounts, up to about 40 percent, of hexane. The marc is then dried to recover the residual hexane. Recovery of the hexane from the miscella and from the marc are both energy intensive processes and require extensive capital equipment.
As previously discussed, rapeseed has been processed on soy bean equipment; it must be ground, cooked and rolled into flakes prior to oil extraction. Even then, the process rate in oil seed processing equipment is only marginally cost effective compared with soy bean. Other potential vegetable matter sources of oil include wheat mill feed, amaranth, and rice bran, for example. Wheat mill feed is the material removed from wheat during milling. Twenty percent or more of all the world's wheat ends up as mill feed. Most of this goes into pet food and animal feed. Amaranth, another potential source of oil, known in the midwest as "pig weed", grows wild. The amaranth cultivars are available to anyone through the U.S.D.A. Wheat mill feed and amaranth can both be extruded into a pellet because they contain sufficient amounts of starch and sugars. The pellets can be processed on standard soy bean equipment, but the hexane solvent typically used for extraction also extracts large quantities of green chlorophyll so that the oil is difficult to refine as well as being dark green, almost black in color.
Rice bran requires a particularly specialized process. The rice bran cannot easily be flaked or made into an extrudable pellet because of lack of starch and the presence of sucrose. Rice bran can be extruded if corn starch is added, but the sucrose can caramelize if heated in the presence of air, giving a dark hue to the oil product. Rice oil can be extracted with hexane using an extractor of the type used to process soy beans, but this process is comparatively slow, energy intensive and expensive when compared to soy beans. Moreover, the remaining rice bran flour still has substantial oil in it.
The present invention provides a very economical method and apparatus for extracting oil from vegetable matter, providing cost advantages over currently utilized techniques. Oil is extracted in a fraction of the time required for previously used commercial methods, and to a substantially greater degree. By use of the process of the present invention, rice bran can be produced which has an oil content of less than five percent, and has no objectionable odor or taste, so that it is suitable for use in human food products. Further, the present invention results in the production of high quality oils at a cost substantially less than is possible with present commercial processes.